Internet-Draft EIGRP February 2013
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Internet-Draft EIGRP February 20131 Introduction
This document describes the Enhanced Interior Gateway Routing Protocol
(EIGRP), routing protocol designed and developed by Cisco Systems. The
convergence technology is based on research conducted at SRI
International. The Diffusing Update Algorithm (DUAL) is the algorithm
used to obtain loop-freedom at every instant throughout a route
computation[3]. This allows all routers involved in a topology change
to synchronize at the same time, which routers not affected by topology
changes are not involved in the recalculation. This document describes
the protocol that implements these functions.
2 Terminology
The following list describes acronyms and definitions for terms used
throughout this document:
EIGRP
Enhanced Interior Gateway Routing Protocol.
Active state
A route that is currently in an unresolved or un-converged
state. The term active is used because the router is actively
attempting to compute an SDAG.
Address Family Identifier (AFI)
A term used to describe an address encoding in a packet. An
address family currently pertains to an IPv4 or IPv6 address.
See [RFC3232] for details.
Autonomous System(AS)
A routing sub-domain representing a logical set of network
segments and attached devices.
Base Topology
The topology associated with the default (none-VRF), routing table.
Downstream Router
A router that is one or more hops away in the direction of the
destination of the information.
Diffusing UPDATE Algorithm(DUAL)
A loop-free routing algorithm used with distance vectors or link
states that provides a diffused computation of a routing table.
It works very well in the presence of multiple topology changes
with low overhead. The technology was researched and developed
at SRI International.
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Feasibility Condition
The feasibility condition is met when the minimum of all neighbors
costs plus the link cost to that neighbor is found, and the
neighbors advertised cost is less than the current successors cost.
This is the Source Node Condition (SNC) sited in reference [2].
Feasible Successor
A neighbor router that meets the feasibility condition.
Neighbor / Peer
Two routers connected to each other with a common network are known
as adjacent neighbors. Neighbors dynamically discover each other
and exchange EIGRP protocol messages. Each router keeps a topology
table containing information learned from each of its neighbors.
Passive state
A route is considered in passive state when there are one or more
minimal cost feasible successors that can reach a destination. The
term passive is used because the router is not actively computing a
shortest path SDAG for this destination. A route in passive state
is usable for forwarding data packets.
PE Router / Provider Edge Router
This is the device that logically sits on the provider side of
the provider/customer demarcation in a network topology.
Routing Information Base(RIB) / Routing Table
A table where a router stores network destinations associated
with a next-hop to reach particular network destinations and the
metric associated with the route.
Subsequent-Address Family Identifier(SAFI)
Unicast and Multicast are examples of a Subsequent-Address
Family Identifier.
Successor Directed Acyclic Graph(SDAG)
When a route to a destination becomes unreachable, it is required
that a router computes a directed graph with respect to the
destination. This decision requires the router to select from the
neighbor topology table a feasible successor.
Sub-Topology
A subset of routes from the base topology. A topology whose
purpose is to implement some user-defined service. The Sub-
Topology is a child of the base topology.
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Successor
The unique neighboring router that has met the feasibility
condition and has been selected as the next-hop for forwarding
packets.
Topology Identifier(TID)
A number that is used to mark prefixes as belonging to a specific
sub-topology.
Type, Length, Value (TLV)
An encoding format used by EIGRP. Each attribute present in a
routing packet is tagged. The tag determines the type and length of
information in the value portion of the attribute. This format
allows extensibility and backward compatibility
Upstream Router
Any router that is one or multiple hops in the direction of the
source of the information.
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Internet-Draft EIGRP February 20133 The DUAL Diffusing Update Algorithm
The Diffusing Update Algorithm (DUAL) provides a loop-free path through
a network made up of nodes and edges (routers and links) at every
instant throughout a route computation. This allows all routers
involved in a topology change to synchronize at the same time. Routers
that are not affected by topology changes are not involved in the
recalculation. The convergence time with DUAL rivals that of any other
existing routing protocol.
3.1 Algorithm Description
The Diffusing Update Algorithm (DUAL) is used by EIGRP to achieve fast
loop-free convergence with little cost in overhead, allowing EIGRP to
provide convergence rates comparable, and in some cases better than,
most common link state protocols[7]. In addition, only nodes that are
affected by a topology change take corrective action which allows DUAL
to have good scaling properties, reduced overhead, and lower complexity
than other IGP protocols, and requiring less information to be
propagated.
Distributed routing algorithms are required to propagate information as
well as coordinate information among all nodes in the network. Unlike
Bellman-Ford distance vector protocols, DUAL uses an approach to
propagation of routing information with feedback known as diffusing
computations. The diffusing computation grows by including nodes that
are affected by the topology change and shrinks by excluding ones that
are not. This allows the computation to dynamically adjust in scope and
terminate as soon as possible.
3.2 Route States
A topology table entry for a destination can have one of two states,
Passive and Active. A route transitions its state when there is a
topology change in the network. This can be caused by link failure,
node failure, or a link cost increase. The two states are as follow:
o Passive
A route is considered in the Passive state when a router is not
performing a route recalculation. When a route is in passive state
it is usable and the next hop is perceived to be downstream of the
destination.
o Active
A destination is in Active state when a router is computing a
Successor Directed Acyclic Graph (SDAG) for the destination.
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While a router has a route in active state, it records the new metric
information but does not make any routing decisions until it goes back
to passive state. A route goes from active state to passive state when
a router receives responses from all of its neighbors and the diffusing
computation is complete.
If an alternate loop free path exists for the route, the neighbor WILL
NOT go into the Active state avoiding a route recalculation. When there
are no feasible successors, a route goes into Active state and a route
recalculation must occur.
3.3 Feasibility Condition
The feasibility condition is a part of DUAL that allows the diffused
computation to terminate as early as possible. Nodes that are not
affected by the topology change are not required to perform a DUAL
computation and may not be aware a topology change occurred. If
informed about a topology change, a router may keep a route in passive
state if it is aware of other paths that are downstream towards the
destination (routes meeting the feasibility condition). A route that
meets the feasibility condition is determined to be loop-free and
downstream along the path between the router and the destination.
In order to facilitate describing the feasibility condition, a few
definitions are in order.
o A Successor for a given route is the next-hop used to forward data
traffic for a destination. Typically the successor is chosen based
on the least cost path to reach the destination.
o A Feasible Successor is a neighbor that meets the feasibility
condition. A feasible successor is regarded as a downstream
neighbor towards the destination but it may not be the least cost
path, but could still be used for forwarding data packets in the
event equal or unequal cost load sharing was active. A feasible
successor can become a successor when the current successor
becomes unreachable.
The Feasibility Condition is met when a neighbor's advertised cost to a
destination is less than the cost of that same destination through the
current successor (or best path). A neighbor that advertises a route
with a cost that does not meet the feasibility condition may be
upstream and thus cannot be guaranteed to be the next hop for a loop
free path. Routes advertised by upstream neighbors are not recorded in
the routing table but saved in a topology table.
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Internet-Draft EIGRP February 20133.4 DUAL Message Types
The Dual algorithm operates with three basic message types, Queries,
Updates, and Replies:
o UPDATE - sent to indicate a change in metric or an addition of a
destination.
o QUERY - sent when a destination becomes unreachable, or the metric
increases to a value greater than its current Feasible Distance.
o REPLY - sent in response to a QUERY or SIA-QUERY
When in passive state, a received query may be propagated if there are
no feasible successors found. If a feasible successor is found, the
query is not propagated and a reply is sent for the destination with a
metric equal to the current routing table metric. When a query is
received in active state a reply is sent and the query is not
propagated. The reply for the destination contains a metric equal to
the current routing table metric.
3.5 Dual Finite State Machine (FSM)
The DUAL finite state machine embodies the decision process for all
route computations. It tracks all routes advertised by all neighbors.
The distance information, known as a metric, is used by DUAL to select
efficient loop free paths. DUAL selects routes to be inserted into a
routing table based on feasible successors. A successor is a
neighboring router used for packet forwarding that has least cost path
to a destination that is guaranteed not to be part of a routing loop.
When there are no feasible successors but there are neighbors
advertising the destination, a recalculation must occur to determine a
new successor.
The amount of time it takes to calculate the route impacts the
convergence time. Even though the recalculation is not processor-
intensive, it is advantageous to avoid recalculation if it is not
necessary. When a topology change occurs, DUAL will test for feasible
successors. If there are feasible successors, it will use any it finds
in order to avoid any unnecessary recalculation.
The finite state machine, which applies per destination in the routing
table, operates independently for each destination. It is true that if
a single link goes down, multiple routes may go into active state.
However, a separate Successor Directed Acyclic Graph (SDAG) is computed
for each destination, so loop-free topologies can be maintained. Figure
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The following describes in detail the state/event/action transitions of
the DUAL FSM. For all steps, the topology table is updated with the new
metric information from either; QUERY, REPLY, or Update is received.
(1) A QUERY is received from a neighbor that is not the current
successor. The route is currently in passive state. A feasible
successor exists since the successor was not affected, so the route
remains in passive state. Since a feasible successor exists, a REPLY is
required to be sent back to the originator of the QUERY.
(2) A directly connected interface has gone up or down, or the metrics
have been changed. Or similarly, an update has been received with a
metric change for an existing destination. If the current successor is
not affected by the change, the route stays in passive state. If the
current successor is no longer reachable, but there is a feasible
successor, the route stays in passive state. In either case, an update
is sent with the new metric information, if it had changed.
(3) A QUERY was received from a neighbor who is the current successor
and no feasible successors exist. The route for the destination goes
into active state. A QUERY is sent to all neighbors on all interfaces.
The QUERY origin flag is set to indicate the QUERY originated from a
neighbor marked as successor for route. The REPLY status flag is set to
1 for all neighbors to indicate outstanding replies.
(4) A directly connected link has gone down or its cost has increased,
or an update has been received with a metric increase. The route to the
destination goes to active state if there are no feasible successors
found. A QUERY is sent to all neighbors on all interfaces. The QUERY
origin flag is to indicate that the router originated the QUERY. The
REPLY status flag is set to 1 for all neighbors to indicate outstanding
replies.
(5) While a route for a destination is in active state and a QUERY is
received from the current successor, the route remains active. The
QUERY origin flag is set to indicate that there was another topology
change while in active state. This indication is used so new feasible
successors are compared to the old metric associated with the current
successor.
(6) While a route for a destination is in active state and a QUERY is
received from a neighbor that is not the current successor, a REPLY
should be sent to the neighbor. The metric advertised in the QUERY
should be recorded.
(7) If a link cost change or an update with a metric change is received
in active state, the router stays in active state for the destination.
The metric information in the update is recorded. When a route is in
the active state, a QUERY and UPDATE is never sent.
(8) If a REPLY for a destination, in active state, is received from a
neighbor or the link between a router and the neighbor fails, the
router records that the neighbor replied to the QUERY. The REPLY status
flag is set to 0 to indicate this. The route stays in active state if
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there are more replies pending. The router has not heard from all
neighbors.
(9) If a route for a destination is in active state, and a link fails
or a cost increase occurred between a router and its successor, the
router treats this case like it has received a REPLY from its
successor. When this occurs after the router originates a QUERY, it
sets QUERY origin flag to indicate that another topology change
occurred in active state.
(10) If a route for a destination is in active state, and a link fails
or a cost increase occurred between a router and its successor, the
router treats this case like it has received a REPLY from its
successor. When this occurs after a neighbor originated a QUERY, the
router sets the QUERY origin flag to indicate that another topology
change occurred in active state.
(11) If a route for a destination is in active state and a link cost
increase to the successor occurred, and the last REPLY was received
from all neighbors, but there is no feasible successor, the route
should stay in active state. A QUERY is sent to all neighbors. The
QUERY origin flag is set to 1.
(12) If a route for a destination is in active state because of a QUERY
received from the current successor, and the last REPLY was received
from all neighbors, but there is no feasible successor, the route
should stay in active state. A QUERY is sent to all neighbors. The
QUERY origin flag is set to 3.
(13) Received replies from all neighbors. Since the QUERY origin flag
indicates the successor originated the QUERY, it transitions to passive
state and sends a REPLY to the old successor.
(14) Received replies from all neighbors. Since the QUERY origin flag
indicates a topology change to the successor while in active state, it
need not send a REPLY to the old successor. The route state transitions
to passive because the feasibility condition is met.
(15) Received replies from all neighbors. Since the QUERY origin flag
indicates either the router itself originated the QUERY or there was a
topology change to the successor while in active state, it need only
send a REPLY to the old successor if the link to it still exists. The
route state transitions to passive because the feasibility condition is
met.
(16) If a route for a destination is in active state because of a QUERY
received from the current successor, the last REPLY was received from
all neighbors, and a feasible successor exists for the destination, the
route can go into passive state.
3.6 DUAL Operation - Example Topology
The following topology (Figure 2) will be used to provide an example of
how DUAL is used to reroute after a link failure. Each node is labeled
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with its costs to destination N. The arrows indicate the successor
(next-hop) used to reach destination N. The least cost path is
selected.
N
|
(1)A ---<--- B(2)
| |
^ |
| |
(2)D ---<--- C(3)
Figure 2 - Stable Topology
Now consider the case where the link between A and D fails (Figure 3).
Only observing destination provided by node N, D enters the active
state and sends a QUERY to all its neighbors, in this case node C. C
determines that it has a feasible successor and replies immediately
with metric 3. C changes its old successor of D to its new single
successor B and the route to N stays in passive state. D receives the
REPLY and can transition out of active state since it received replies
from all its neighbors. D now has a viable path to N through C. D
elects C as its successor to reach node N with a cost of 4. Note that
node A and B were not involved in the recalculation since they were not
affected by the change.
N N
| |
A ---<--- B A ---<--- B
| | | |
X | ^ |
| | | |
D ---<--- C D ---<--- C
Q-> <-R
N
|
(1)A ---<--- B(2)
|
^
|
(4)D --->--- C(3)
Figure 3 - Link between A and D fails
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Let's consider the situation in Figure 4, where feasible successors may
not exist. If the link between node A and B fails, B goes into active
state for destination N since it has no feasible successors. Node B
sends a QUERY to node C. C has no feasible successors, so it goes
active for destination N and sends QUERY to B. B replies to the QUERY
since it is in active state. Once C has received this reply, it has
heard from all its neighbors, so it can go passive for the unreachable
route. As C removes the (now unreachable) destination from its table, C
sends REPLY to its old successor. B receives this reply from C, and
determines this is the last REPLY it is waiting on before determining
what the new state of the route should be; on receiving this reply, B
deletes the route to N from its routing table. Since B was the
originator of the initial QUERY it does not have to send a REPLY to its
old successor (it would not be able to any ways, because the link to
its old successor is down). Note that nodes A and D were not involved
in the recalculation since their successors were not affected.
N N
| |
(1)A ---<--- B(2) A ------- B Q
| | | | | ^ ^
^ ^ ^ | | | |
| | | | v | |
(2)D C(3) D C Ack R
Figure 4
No Feasible Successors when link between A and B fails
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Internet-Draft EIGRP February 20134 EIGRP Packets
EIGRP uses 5 different packet types to operate.
o HELLO/Ack Packets
o QUERY Packets
o UPDATE Packets
o REPLY Packets
EIGRP packets will be encapsulated in the respective network layer
protocol that it is supporting. Since EIGRP is potentially capable of
running in an integrated mode the encapsulation is not specified.
Support for network layer protocol fragmentation is supported, though
EIGRP will attempt to avoid maximum size packets that exceed the
interface MTU by sending multiple packets which are less than or equal
to MTU sized packets.
Each packet transmitted will use either multicast or unicast network
layer destination addresses. When multicast addresses are used a
mapping for the data link multicast address (when available) must be
provided. The source address will be set to the address for the sending
interface, if applicable. The following network layer multicast
addresses and associated data link multicast addresses will be used.
- IPv4 - 224.0.0.10
- IPv6 - FF02:0:0:0:0:0:0:A
The above data link multicast addresses will be used on multicast
capable media, and will be media independent for unicast addresses.
Network layer addresses will be used and the mapping to media addresses
will be achieved by the native protocol mechanisms.
4.1 UPDATE Packets
UPDATE packets are used to convey destinations, and the reachability of
the destinations. When a new neighbor is discovered, unicast UPDATE
packets are used to transmit a full table to the new neighbor, so the
neighbor can build up its topology table. In normal operation (other
than neighbor startup such as a link cost changes), UPDATE packets are
multicast. UPDATE packets are always transmitted reliably. Each TLV
destination will be processed individually through the DUAL state
machine.
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Internet-Draft EIGRP February 20134.2 QUERY Packets
A QUERY packet sent by a router advertises that a route is in active
state and the originator is requesting alternate path information from
its neighbors. An infinite metric is encoded by setting the Delay part
of the metric to its maximum value. If there is a topology change that
causes multiple destinations to go unreachable, EIGRP will build a
single QUERY packet with all destinations present. The state of each
route is recorded individually, so a responding QUERY or REPLY need not
contain all the same destinations in a single packet. Since the packets
are guaranteed reliable all route QUERY packets are guaranteed
reliable.
When a QUERY packet is received, each destination will trigger a DUAL
event and the state machine will run individually for each route. Once
the entire original QUERY packet is processed, than a REPLY or SIA-
REPLY will be sent with the latest information.
4.3 REPLY Packets
A REPLY packet will be sent in response to a QUERY or SIA-QUERY packet,
if the router believes it has an alternate feasible successor. The
REPLY packet will include a TLV for each destination and the associated
victimized metric in its own topology table. The REPLY packet is sent
after the entire received QUERY packet is processed.
When a REPLY packet is received, there is no reason to process the
packet before an acknowledgment is sent. Therefore, an Ack packet is
sent immediately and then the packet is processed. Each TLV destination
will be processed individually through the DUAL state machine.
4.4 Exception Handling4.4.1 Active Route Duration control
When an EIGRP router transitions to ACTIVE state for a particular
destination a QUERY is sent to all neighbors and the ACTIVE timer is
started to limit the amount of time a destination may remain in an
active state. The default time DUAL is allowed to stay active, trying
to resolve a path to a destination, is a maximum of six (6) minutes.
This is broken into an initial 90 seconds period following the QUERY,
and up to 3 additional "busy" periods in which a SIA-QUERY is sent.
Failure to respond to a SIA-QUERY with in the 90 second will result in
the neighbor being declared in an Stuck In Active (SIA) state.
4.4.2 Stuck-in-Active
A route is regarded as Stuck-In-Active (SIA) when DUAL does not
receive
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a reply to the active process. This process is begun when a QUERY is
sent by. After the initial 90 seconds, the router will send a SIA-
QUERY, this must be replied to with either a REPLY or SIA-REPLY.
Failure of a neighbor to send either a REPLY or SIA-REPLY with-in the
90 seconds will result in the neighbor being deemed to be in an SIA
state. If the SIA state is declared, DUAL will then delete all routes
from that neighbor, acting as if the neighbor had responded with an
unreachable message for all routes.
4.4.3 SIA-QUERY
When a QUERY is still outstanding and awaiting a REPLY from a neighbor,
there is insufficient information to determine why a REPLY has not been
received. A lost packet, congestion on the link, or a slow neighbor
could cause a lack of REPLY from a downstream neighbor. In order to
attempt to ascertain if the neighbor device is still attempting to
converge on the active route, an EIGRP router MAY send a SIA-QUERY
packet to the active neighbors. This enables an EIGRP router to
determine if there is a communication issue with the neighbor, or it is
simply still attempting to converge with downstream routers. By
sending a SIA-QUERY, the originating router may extend the effective
active time by resetting the Active timer which has been previously set
and thus allow convergence to continue so long as neighbor devices
successfully communicate that convergence is still underway.
The SIA-QUERY packet SHOULD be sent on a per-destination basis at one-
half of the Active timeout period. Up to three SIA-QUERY packets for a
specific destination may be sent, each at a value of one-half the
Active time, so long as each are successfully acknowledged and met with
a SIA-REPLY.
Upon receipt of a SIA-QUERY packet, and EIGRP router should first send
an ACK and then continue to process the SIA-QUERY information. The
QUERY is sent on a per-destination basis at approximately one-half the
active time. If the EIGRP router is still active for the destination
specified in the SIA-QUERY, the router SHOULD respond to the originator
with the SIA-REPLY indicating that active processing for this
destination is still underway by setting the Active flag in the packet
upon response.
If the router receives a SIA-QUERY referencing a destination for which
it has not received the original QUERY, the router SHOULD treat the
packet as though it was a standard QUERY:
1) Acknowledge the receipt of the packet
2) Send a REPLY if a Successor exists
3) If the QUERY is from the successor, transition to the Active
state and send a SIA-REPLY with the Active bit set
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Internet-Draft EIGRP February 20134.4.4 SIA-REPLY
A SIA-REPLY packet is the corresponding response upon receipt of a SIA-
QUERY from an EIGRP neighbor. The SIA-REPLY packet will include a TLV
for each destination and the associated metric for which is stored in
its own routing table. The SIA-REPLY packet is sent after the entire
received SIA-QUERY packet is processed.
If the EIGRP router is still ACTIVE for a destination, the SIA-REPLY
packet will be sent with the ACTIVE bit set. This confirms for the
neighbor device that the SIA-QUERY packet has been processed by DUAL
and that the router is still attempting to resolve a loop-free path
(likely awaiting responses to its own QUERY to downstream neighbors).
The SIA-REPLY informs the recipient that convergence is complete or
still ongoing, however; it is an explicit notification that the router
is still actively engaged in the convergence process. This allows the
device that sent the SIA-QUERY to determine whether it should continue
to allow the routes that are not converged to be in the ACTIVE state,
or if it should reset the neighbor relationship and flush all routes
through this neighbor.
5 EIGRP Protocol Operation
EIGRP has four basic components:
o Finite State Machine
o Reliable Transport Protocol
o Neighbor Discovery/Recovery
o Route Management
5.1 Finite State Machine
The detail of DUAL, the State Machine used by EIGRP is covered in
section 35.2 Reliable Transport Protocol
The reliable transport is responsible for guaranteed, ordered delivery
of EIGRP packets to all neighbors. It supports intermixed transmission
of multicast or unicast packets. Some EIGRP packets must be transmitted
reliably and others need not. For efficiency, reliability is provided
only when necessary. For example, on a multi-access network that has
multicast capabilities, such as Ethernet, it is not necessary to send
HELLOs reliably to all neighbors individually. EIGRP sends a single
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multicast HELLO with an indication in the packet informing the
receivers that the packet need not be acknowledged. Other types of
packets, such as UPDATE packets, require acknowledgment and this is
indicated in the packet. The reliable transport has a provision to send
multicast packets quickly when there are unacknowledged packets
pending. This helps insure that convergence time remains low in the
presence of varying speed links.
The DUAL Algorithm assumes there is lossless communication between
devices and thus must rely upon the transport protocol to guarantee
that messages are transmitted reliably. EIGRP implements the Reliable
Transport Protocol to ensure ordered delivery and acknowledgement of
any messages requiring reliable transmission. State variables such as a
received sequence number, acknowledgment number, and transmission
queues MUST be maintained on a per neighbor basis.
The following sequence number rules must be met for the reliable EIGRP
protocol to work correctly:
o A sender of a packet includes its global sequence
number in the sequence number field of the fixed
header. The sender includes the receivers sequence
number in the acknowledgment number field of the fixed
header.
o Any packets that do not require acknowledgment must be
sent with a sequence number of 0.
o Any packet that has an acknowledgment number of 0
indicates that sender is not expecting to explicitly
acknowledging delivery. Otherwise, it is acknowledging
a single packet.
o Packets that are network layer multicast must contain
acknowledgment number of 0.
When a router transmits a packet, it increments its sequence number and
places mark the packet as requiring acknowledgment by all neighbors on
the interface for which the packet is sent. When individual
acknowledgments are unicast addressed by the receivers to the sender
with the acknowledgment number equal to the packets sequence number,
the sender SHALL clear the pending acknowledgement requirement for the
packet from the respective neighbor. If the required acknowledge is not
received for the packet, it MUST be retransmitted. Retransmissions will
occur for a maximum of 5 seconds1.
The protocol has no explicit windowing support. A receiver will
acknowledge each packet individually and will drop packets that are
received out of order. Duplicate packets are also discarded upon
receipt. Acknowledgments are not accumulative. Therefore an ACK with a
non-zero sequence number acknowledges a single packet.
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There are situations when multicast and unicast packets are transmitted
close together on multi-access broadcast capable networks. The reliable
transport mechanism MUST assure that all multicasts are transmitted in
order as well as not mixing the order among unicasts and multicast
packets. The reliable transport provides a mechanism to deliver
multicast packets in order to some receivers quickly, while some
receivers have not yet received all unicast or previously sent
multicast packets. The SEQUENCE_TYPE TLV in HELLO packets achieves
this. This will be explained in more detail in this section.
Figure 5 illustrates the reliable transfer protocol on point-to-point
links. There are two scenarios that may occur, an UPDATE initiated
packet exchange, or a QUERY initiated packet exchange. This example
will assume no packet loss.
Router A Router B
An UPDATE Exchange
<----------------
UPDATE (multicast)
A receives packet Seq=100, Ack=0
Queues pkt on A's retrans list
---------------->
ACK (unicast)
Seq=0, Ack=100 Receives Ack
Process Update Dequeue pkt from A's retrans list
A QUERY Exchange
<----------------
QUERY (multicast)
A receives packet Seq=101, Ack=0
Process QUERY Queues pkt on A's retrans list
---------------->
REPLY (unicast)
Seq=201, Ack=101 Process Ack
Dequeue pkt from A's retrans list
Process REPLY pkt
<----------------
ACK (unicast)
A receives packet Seq=0, Ack=201
Figure 5 - Reliable Transfer on point-to-point links
The UPDATE exchange sequence requires UPDATE packets sent to be
delivered reliably. The UPDATE packet transmitted contains a sequence
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---------------->
C send ACK (unicast)
Seq=0, Ack=101
---------------->
D send ACK (unicast)
Seq=0, Ack=101
<----------------
A resends UPDATE (unicast to B)
Seq=100, Ack=0
B Packet duplicate
--------------->
B sends ACK (unicast) A removes pkt from retrans list
Seq=0, Ack=100
<----------------
A resends UPDATE (unicast to B)
Seq=101, Ack=0
--------------->
B sends ACK (unicast) A removes pkt from retrans list
Seq=0, Ack=101
Figure 9
Initially Router-A sends a multicast addressed UPDATE packet on the
LAN. B and C receive it and send acknowledgments. Router-B receives the
UPDATE but the acknowledgment sent is lost on the network. Before the
retransmission timer for Router-B's packet expires, there is an event
that causes a new multicast addressed UPDATE to be sent. Router-A
detects that there is at least one neighbor on the interface with a
full queue. Therefore, it is REQUIRED to tell that neighbor to not
receive the next packet or it would receive it out of order. Router-A
builds a HELLO packet with a SEQUENCE_TYPE TLV indicating all the
neighbors that have full queues. In this case, the only neighbor
address in the list is Router-B. The HELLO packet is multicasted
unreliably out the interface. Router-C and Router-D process the
SEQUENCE_TYPE TLV by looking for its own address in the list. If it is
not found, they put themselves in Conditionally Received (CR-mode)
mode. Any subsequent packets received that have the CR-flag set can be
received. Router-B does not put itself in CR-mode because it finds
itself in the list. Packets received by Router-B with the CR-flag MUST
be discarded and not acknowledged. Later, Router-A will unicast
transmit both packets 100 and 101 directly to Router-B. Router-B
already has 100 so it discards and acknowledges it. Router-B then
accepts packet 101 and acknowledges it too. Router-A can remove both
packets off Router-B's transmission list.
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Internet-Draft EIGRP February 20135.2.1 Bandwidth on Low-Speed Links
By default, EIGRP limits itself to using no more than 50% of the
bandwidth reported by an interface when determining packet-pacing
intervals. If the bandwidth does not match the physical bandwidth (the
network architect may have put in an artificially low or high bandwidth
value to influence routing decisions), EIGRP may:
1. Generate more traffic than the interface can handle, possibly
causing drops, thereby impairing EIGRP performance.
2. Generate a lot of EIGRP traffic that could result in little
bandwidth remaining for user data.
5.3 Neighbor Discovery/Recovery
Neighbor Discovery/Recovery is the process that routers use to
dynamically learn of other routers on their directly attached networks.
Routers MUST also discover when their neighbors become unreachable or
inoperative. This process is achieved with low overhead by periodically
sending small HELLO packets. As long as any packets are received from a
neighbor, the router can determine that neighbor is alive and
functioning. Only after a neighbor router is considered operational can
the neighboring routers exchange routing information.
5.3.1 Neighbor HoldTime
Each router keeps state information about adjacent neighbors. When
newly discovered neighbors are learned the address, interface, and hold
time of the neighbor is noted. When a neighbor sends a HELLO, it
advertises its HoldTime. The HoldTime is the amount of time a router
treats a neighbor as reachable and operational. In other words, if a
HELLO packet isn't heard within the HoldTime, then the HoldTime
expires. When the HoldTime expires, DUAL is informed of the topology
change.
5.3.2 HELLO Packets
When an EIGRP router is initialized, it will start sending HELLO
packets out any interface for which EIGRP is enabled. HELLO packets,
when used for neighbor discovery, are normally sent multicast
addressed. The HELLO packet will include the configured EIGRP metric K-
values. Two routers become neighbors only if the K-values are the same.
This enforces that the metric usage is consistent throughout the
Internet. Also included in the HELLO packet, is a HoldTime value. This
value indicates to all receivers the length of time in seconds that the
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neighbor is valid. The default HoldTime will be 3 times the HELLO
interval. HELLO packets will be transmitted every 5 seconds (by
default). There MAY be a configuration command that controls this value
and therefore changes the HoldTime. HELLO packets are not transmitted
reliably so the sequence number should be set to 0.
5.3.3 UPDATE Packets
When a router detects a new neighbor by receiving a HELLO packet from a
neighbor not presently known, it will send a unicast UPDATE packet to
the neighbor with no routing information. The initial UPDATE sent MUST
have the INIT-flag set. This instructs the neighbor to advertise its
routes. The INIT-flag is also useful when a neighbor goes down and
comes back up before the router detects it went down. In this case, the
neighbor needs new routing information. The INIT-flag informs the
router to send it.
5.3.4 Initialization Sequence Router A Router B
(just booted) (up and running)
(1)---------------->
HELLO (multicast) <---------------- (2)
Seq=0, Ack=0 UPDATE (unicast)
Seq=10, Ack=0, INIT
(3)----------------> UPDATE 11 us queued
UPDATE (unicast)
Seq=100, Ack=10, INIT <---------------- (4)
UPDATE (unicast)
Seq=11, Ack=100
All UPDATES sent
(5)--------------/lost/->
ACK (unicast)
Seq=0, Ack=11
(5 seconds later)
<---------------- (6)
Duplicate received, UPDATE (unicast)
Packet discarded Seq=11, Ack=100
(7)--------------->
ACK (unicast)
Seq=0, Ack=11
Figure 9 - Initialization Sequence
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(1) Router A sends multicast HELLO and Router B discovers it.
(2) Router B detects new neighbor and downloads its routing table to
Router A. The number of destinations in its routing table will
require 2 UPDATE packets to be sent. The first UPDATE is sent with
the INIT-Flag to request A to send its routing table information. The
second packet is queued, and cannot be sent until the first is
acknowledged.
(3) Router A receives first UPDATE and processes it as a DUAL event.
Stores information in topology table and possibly the routing table.
Sends its first and only UPDATE packet with an accompanied Ack.
(4) Router B receives UPDATE packet 100 from Router A. Router B can
dequeue packet 10 from A's transmission list since the UPDATE
acknowledged 10. It can now send UPDATE packet 11 and with an
acknowledgment of Router A's UPDATE.
(5) Router A receives the last UPDATE from Router B and acknowledges
it. The acknowledgment gets lost.
(6) Router B later retransmits the UPDATE to Router A.
(7) Router A detects the duplicate and simply acknowledges the
packet. Router B dequeues packet 11 from A's transmission list and
both routers are up and synchronized.
5.3.5 QUERY Packets During Neighbor Formation
As described above, during the initial formation of the neighbor
relationship, EIGRP uses a form of three-way handshake to verify both
unicast and multicast connectivity are working successfully. During
this period of neighbor creation the new neighbor is considered the
pending state, and is not eligible to be included in the convergence
process. Because of this, any QUERY received by an EIGRP router would
not cause a QUERY to be sent to the new (and pending) neighbor. It
would perform the DUAL process without the new peer in the
conversation.
To do this, when a router in the process of establishing a new neighbor
receives a QUERY from a fully established neighbor, it performs the
normal DUAL Feasible Successor check to determine whether it needs to
REPLY with a valid path or whether it needs to enter the Active process
on the prefix.
If it determines that it must go active, each fully established
neighbor that participates in the convergence process will be sent a
QUERY packet and REPLY packets are expected from each. Any pending
neighbor will not be expected to REPLY and will not be sent a QUERY
directly. If it resides on an interface containing a mix of fully
established neighbors and pending neighbors, it might receive the QUERY
but will not be expected to REPLY to it.
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Internet-Draft EIGRP February 20135.3.6 Neighbor Formation
To prevent packets from being sent to a neighbor prior to the multicast
and unicast delivery has been verified as reliable, a 3-way handshake
is utilized.
During normal adjacency formation, multicast HELLOs cause the EIGRP
process to place new neighbors into the neighbor table. Unicast packets
are then used to exchange known routing information, and complete the
neighbor relationship (section 5.2)
To prevent EIGRP from forming sending sequenced packets to neighbor
which fail to have bidirectional unicast/multicast, or one neighbor
restarts while building the relationship, EIGRP SHALL place the newly
discovered neighbor in a "pending" state as follows:
o When Router-A receives the first multicast HELLO from Router-B,
it places Router-B in the pending state, and transmits a unicast
UPDATE containing no topology information and SHALL set the
initialization bit
o While Router-B is in this state, A will not send it any a QUERY
or UPDATE
o When Router-A receives the unicast acknowledgement from Router-
B, it will check the state from pending to up
5.3.7 Topology Table
The Topology Table is populated by the protocol dependent modules and
acted upon by the DUAL finite state machine. It contains all
destinations advertised by neighboring routers. Associated with each
entry are the destination address and a list of neighbors that have
advertised this destination. For each neighbor, the advertised metric
is recorded. This is the metric that the neighbor stores in its routing
table. If the neighbor is advertising this destination, it must be
using the route to forward packets. This is an important rule that
distance vector protocols MUST follow.
Also associated with the destination is the metric that the router uses
to reach the destination. This is the sum of the best-advertised metric
from all neighbors plus the link cost to the best neighbor. This is the
metric that the router uses in the routing table and to advertise to
other routers.
5.3.8 Route Management
EIGRP has the notion of internal and external routes. Internal routes
are ones that have been originated within an EIGRP autonomous system
(AS). Therefore, a directly attached network that is configured to run
EIGRP is considered an internal route and is propagated with this
information throughout the network topology.
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External routes are destinations that have been learned though another
source, such as a routing protocol or static route. These routes are
marked individually with the identity of their origination.
External routes are tagged with the following information:
o The router ID of the EIGRP router that redistributed the route.
o The AS number where the destination resides.
o A configurable administrator tag.
o Protocol ID of the external protocol.
o The metric from the external protocol.
o Bit flags for default routing.
As an example, suppose there is an AS with three border routers. A
border router is one that runs more than one routing protocol. The AS
uses EIGRP as the routing protocol. Two of the border routers, BR1 and
BR2, also use Open Shortest Path First (OSPF) and the other, BR3, also
uses Routing Information Protocol (RIP).
Routes learned by one of the OSPF border routers, BR1, can be
conditionally redistributed into EIGRP. This means that EIGRP running
in BR1 advertises the OSPF routes within its own AS. When it does so,
it advertises the route and tags it as an OSPF learned route with a
metric equal to the routing table metric of the OSPF route. The router-
id is set to BR1. The EIGRP route propagates to the other border
routers. Let's say that BR3, the RIP border router, also advertises the
same destinations as BR1. Therefore BR3, redistributes the RIP routes
into the EIGRP AS. BR2, then, has enough information to determine the
AS entry point for the route, the original routing protocol used, and
the metric. Further, the network administrator could assign tag values
to specific destinations when redistributing the route. BR2 can use any
of this information to use the route or re-advertise it back out into
OSPF.
Using EIGRP route tagging can give a network administrator flexible
policy controls and help customize routing. Route tagging is
particularly useful in transit AS's where EIGRP would typically
interact with an inter-domain routing protocol that implements more
global policies.
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Internet-Draft EIGRP February 20135.4 EIGRP Metric Coefficients
EIGRP allows for modification of the default composite metric
calculation though the use of coefficients (K values). This adjustment
allows for per-deployment tuning of network behavior. Setting K values
up to 254 scales the impact of the scalar metric on the final composite
metric.
EIGRP defaults coefficients have been carefully selected to provide
optimal performance in most networks. The default K values are
K1 == K3 == 1
K2 == K4 == K5 == 0
K6 == 0
If K5 is equal to 0 then reliability quotient is defined to be 1.
5.4.1 Coefficients K1 and K2
K1 is used to allow path selection to be based on the bandwidth
available along the path. EIGRP can use one of two variations of
Throughput based path selection.
o Maximum Theoretical Bandwidth; paths chosen based on the highest
reported bandwidth
o Network Throughput: paths chosen based on the highest
'available' bandwidth adjusted by congestion-based effects
(interface reported load)
By default EIGRP computes the Throughput using the maximum theoretical
throughput expressed in picoseconds per kilobyte of data sent. This
inversion results in a larger number (more time) ultimately generating
a worse metric.
If K2 is used, the effect of congestion as a measure of load reported by
the interface will be used to simulate the "available throughput by
adjusting the maximum throughput.
5.4.2 Coefficients K3
K3 is used to allow delay or latency-based path selection. Latency and
Delay are similar terms that refer to the amount of time it takes a bit
to be transmitted to an adjacent neighbor. EIGRP uses one-way based
values either provided by the interface, or computed as a factor of the
links bandwidth.
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Internet-Draft EIGRP February 20135.4.3 Coefficients K4 and K5
K4 and K5 are used to allow for path selection based on link quality and
packet loss. Packet loss caused by network problems result in highly
noticeable performance issues or jitter with streaming technologies,
voice over IP, online gaming and videoconferencing, and will affect all
other network applications to one degree or another.
Critical services should pass with less than 1% packet loss. Lower
priority packet types might pass with less than 5% and then 10% for the
lowest of priority of services. The final metric can be weighted based
on the reported link quality.
5.4.4 Coefficients K6
K6 has been introduced with Wide Metric support and is used to allow for
Extended Attributes, which can be used to reflect in a higher aggregate
metric than those having lower energy usage.
Currently there are two Extended Attributes, jitter and energy, defined
in the scope of this document.
5.4.1.1 Jitter
Use of Jitter-based Path Selection results in a path calculation with
the lowest reported jitter. Jitter is reported and the interval between
the longest and shortest packet delivery and is expressed in
microseconds. Higher values results in a higher aggregate metric when
compared to those having lower jitter calculations.
Jitter is measured in microseconds and is accumulated along the path,
with each hop using an averaged 3-second period to smooth out the
metric change rate.
Presently, EIGRP does not currently have the ability to measure jitter,
and as such the default value will be zero (0). Performance based
solutions such as PfR could be used to populate this field.
5.4.1.2 Energy
Use of Energy-based Path Selection results in paths with the lowest
energy usage being selected in a loop free and deterministic manner.The
amount of energy used is accumulative and has results in a higher
aggregate metric than those having lower energy.
Presently, EIGRP does not currently have the ability to measure energy
usage, and as such the default value will be zero (0).
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Internet-Draft EIGRP February 20135.5 EIGRP Metric Calculations5.5.1 Classic Metrics
One of the original goals of EIGRP was to offer and enhance routing
solutions for IGRP. To achieve this, EIGRP used the same composite
metric as IGRP, with the terms multiplied by 256 to change the metric
from 24 bits to 32 bits.
The composite metric is based on bandwidth, delay, load, and
reliability. MTU is not an attribute for calculating the composite
metric.
5.5.1.1 Classic Composite Formulation
EIGRP calculates the composite metric with the following formula:
metric = {K1*BW + [(K2*BW)/(256-load)]+(K3*delay)}*{K5/(reliability+K4)}
In this formula, bandwidth (BW) is the lowest interface bandwidth along
the path, and delay is the sum of all outbound interface delays along
the path. The router dynamically measures reliability and load. It
expresses 100 percent reliability as 255/255. It expresses load as a
fraction of 255. An interface with no load is represented as 1/255.
Bandwidth is the inverse minimum bandwidth (in kbps) of the path in
bits per second scaled by a factor of 256 x 107. The formula for
bandwidth is
bandwidth= (256 x 107)/BWmin
The delay is the sum of the outgoing interface delays (in microseconds)
to the destination. A delay of all 1s (that is, a delay of hexadecimal
FFFFFFFF) indicates that the network is unreachable. The formula for
delay is
delay = [sum of delays] x 256
Reliability is a value between 1 and 255. Cisco IOS routers display
reliability as a fraction of 255. That is, 255/255 is 100 percent
reliability or a perfectly stable link; a value of 229/255 represents a
90 percent reliable link. Load is a value between 1 and 255. A load of
255/255 indicates a completely saturated link. A load of 127/255
represents a 50 percent saturated link.
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Internet-Draft EIGRP February 20135.5.2 Wide Metrics
To accommodate interfaces with high bandwidths, and to allow EIGRP to
perform the path selection; the EIGRP packet and composite metric
formula has been modified to choose paths based on the computed time,
measured in picoseconds, information takes to travel though the links.
5.5.1.3 Wide Metric Vectors
EIGRP uses five 'vector' metrics: minimum throughput, latency, load,
reliability, and maximum transmission unit (MTU). These values are
calculated from destination to source as follows:
o Throughput - Minimum value
o Latency - accumulative
o Load - maximum
o Reliability - minimum
o MTU - minimum
o Hop count - Accumulative
To this there are two additional values: jitter and energy. These two
values are accumulated from destination to source:
o Jitter - accumulative
o Energy - accumulative
These Extended Attributes, as well as any future ones, will be
controlled via K6. If K6 is non-zero, these will be additive to the
path's composite metric. Higher jitter or energy usage will result in
paths that are worse than those which either does not monitor these
attributes, or which have lower values.
EIGRP will not send these attributes if the router does not provide
them. If the attributes are received, then EIGRP will use them in the
metric calculation (based on K6) and will forward them with those
routers values assumed to be "zero" and the accumulative values forward
unchanged.
The use of the vector metrics allows EIGRP to compute paths based on
any of four (bandwidth, delay, reliability, and load) path selection
schemes. The schemes are distinguished based on the choice of the key
measured network performance metric.
Of these vector metric components, by default, only minimum throughput
and latency are traditionally used to compute best path. Unlike most
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metrics, minimum throughput is set to the minimum value of the entire
path, and it does not reflect how many hops or low throughput links are
in the path, nor does it reflect the availability of parallel links.
Latency is calculated based on one-way delays, and is a cumulative
value, which increases with each segment in the path.
Network Designers Note: when trying to manually influence EIGRP path
selection though interface bandwidth/delay configuration, the
modification of bandwidth is discouraged for following reasons:
1. The change will only effect the path selection if the configured
value is the lowest bandwidth over the entire path.
2. Changing the bandwidth can have impact beyond affecting the EIGRP
metrics. For example, quality of service (QoS) also looks at the
bandwidth on an interface.
3. EIGRP throttles to use 50 percent of the configured bandwidth.
Lowering the bandwidth can cause problems like starving EIGRP
neighbors from getting packets because of the throttling back.
Changing the delay does not impact other protocols nor does it cause
EIGRP to throttle back, and because, as it's the sum of all delays, has
a direct effect on path selection.
5.5.1.4 Wide Metric Conversion Constants
EIGRP uses a number of defined constants for conversion and calculation
of metric values. These numbers are provided here for reference
EIGRP_BANDWIDTH 10,000,000
EIGRP_DELAY_PICO 1,000,000
EIGRP_INACCESSIBLE 0xFFFFFFFFFFFFFFFFLL
EIGRP_MAX_HOPS 100
EIGRP_CLASSIC_SCALE 256
EIGRP_WIDE_SCALE 65536
EIGRP_RIB_SCALE 128
When computing the metric using the above units, all capacity
information will be normalized to kilobytes and picoseconds before
being used. For example, delay is expressed in microseconds per
kilobyte, and would be converted to kilobytes per second; likewise
energy would be expressed in power per kilobytes per second of usage.
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Internet-Draft EIGRP February 20135.5.1.5 Throughput Formulation
The formula for the conversion for Max-Throughput value directly from
the interface without consideration of congestion-based effects is as
follows:
(EIGRP_BANDWIDTH * EIGRP_WIDE_SCALE)
Max-Throughput = K1 * ------------------------------------
Interface Bandwidth (kbps)
If K2 is used, the effect of congestion as a measure of load reported by
the interface will be used to simulate the "available throughput by
adjusting the maximum throughput according to the formula:
K2 * Max-Throughput
Net-Throughput = Max-Throughput + ---------------------
M256 - Load
K2 has the greatest effect on the metric occurs when the load increases
beyond 90%.
5.5.1.6 Latency Formulation
Transmission times derived from physical interfaces MUST be n units of
picoseconds, or converted to picoseconds prior to being exchanged
between neighbors, or used in the composite metric determination.
This includes delay values present in configuration-based commands
(i.e. interface delay, redistribute, default-metric, route-map, etc.)
The delay value is then converted to a "latency" using the formula:
Delay * EIGRP_WIDE_SCALE
Latency = K3 * --------------------------
EIGRP_DELAY_PICO
5.5.1.7 Composite Formulation
K5
metric =[(K1*Net-Throughput) + Latency)+(K6*ExtAttr)] * ------
K4=Rel
By default, the path selection scheme used by EIGRP is a combination of
Throughput and Latency where the selection is a product of total
latency and minimum throughput of all links along the path:
metric = (K1 * min(Throughput)) + (K3 * sum(Latency)) }
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Internet-Draft EIGRP February 20136 Security Considerations
By the nature of being promiscuous, EIGRP will neighbor with any router
that sends a valid HELLO packet. Due to security considerations, this
"completely" open aspect requires policy capabilities to limit peering
to valid routers.
EIGRP does not rely on a PKI or a more heavy weight authentication
system. These systems challenge the scalability of EIGRP, which was a
primary design goal.
Instead, DoS attack prevention will depend on implementations rate-
limiting packets to the control plane as well as authentication of the
neighbor though the use of SHA2-256
7 IANA Considerations
This document has no actions for IANA.
8 References8.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, [RFC2119], April 1997.
[2] Crocker, D. and Overell, P.(Editors), "Augmented BNF for
Syntax Specifications: ABNF", [RFC2234], Internet Mail
Consortium and Demon Internet Ltd., November 1997.
[3] A Unified Approach to Loop-Free Routing using Distance Vectors
or Link States, J.J. Garcia-Luna-Aceves, 1989 ACM 089791-332-
9/89/0009/0212, pages 212-223.
[4] Loop-Free Routing using Diffusing Computations, J.J. Garcia-
Luna-Aceves, Network Information Systems Center, SRI
International to appear in IEEE/ACM Transactions on
Networking, Vol. 1, No. 1, 1993.
[5] BGP Extended Communities Attribute [RFC4360]
[6] HMAC-SHA256, SHA384, SHA512 in IPsec [RFC4868]
8.2 Informative References
[7] OSPF Version 2, Network Working Group [RFC1247], J. Moy, July
1991.
9 Acknowledgments
This document was prepared using 2-Word-v2.0.template.dot.
An initial thank you goes to Dino Farinacci, Bob Albrightson, and Dave
Katz. Their significant accomplishments towards the design and
development of the EIGRP protocol provided the bases for this document.
A special and appreciative thank you goes to the core group of Cisco
engineers, whose dedication, long hours, and hard work lead the
evolution of EIGRP over the following decade. They are Donnie Savage,
Mickel Ravizza, Heidi Ou, Dawn Li, Thuan Tran, Catherine Tran, Don
Slice, Claude Cartee, Donald Sharp, Steven Moore, Richard Wellum, Ray
Romney, Jim Mollmann, Dennis Wind, Chris Van Heuveln, Gerald Redwine,
Glen Matthews, Michael Wiebe, and others.
The authors would like to gratefully acknowledge many people who have
contributed to the discussions that lead to the making of this
proposal. They include Chris Le, Saul Adler, Scott Van de Houten,
Lalit Kumar, Yi Yang, Kumar Reddy, David Lapier, Scott Kirby, David
Prall, Jason Frazier, Eric Voit, Dana Blair, Jim Guichard, and Alvaro
Retana
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